US 84 MISSISSIPPI BIOGRAPHY RIVER BRIDGE TRUSS PIN AND ... · Page 1 of 14 US 84 MISSISSIPPI RIVER...

US 84 MISSISSIPPI

RIVER BRIDGE –

TRUSS PIN AND

LINK

REPLACEMENT

JAMES GREGG

JUSTIN WALKER

MICHAEL XIN

BIOGRAPHY

James Gregg is the bridge

department manager for HNTB

Baton Rouge, LA office. He

served as the project manager

for US 84 Mississippi River

Bridge Rehabilitation project as

well as assisted with several

NBIS inspections of the bridge.

Mr. Gregg has over 10 years’

experience with design and

rehabilitation of complex

structures, design-builds, and

bridge construction inspections.

Justin Walker is the current state

bridge engineer for Mississippi

Department of Transportation

with over 15 years of experience

in bridge design. He currently

serves as a member of

AASHTO T17 Welding

Subcommittee on Bridge

structures and is a member of

the Mississippi Engineering

Society and the Structural

Engineering Association of

Mississippi.

Michael Xin is a principal

bridge engineer in HNTB

Chicago, IL office with more

than 20 years’ experience on

complex bridges. Michael

served as one of the lead

designers of the for the US 84

Mississippi River Bridge

Rehabilitation project.

SUMMARY

The US 84 Mississippi River

Bridge is a 5 span cantilever

truss bridge crossing the

Mississippi River in Natchez,

Mississippi. Two lower truss

pins on the bridge shifted

transversely and were flush

with the outside gusset. The

existing truss pins and links

were removed and replaced.

Temporary restraints were

used to bypass the load in the

truss pins and link and

instrumentation used to

evaluate stresses in the truss

during removal.

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US 84 MISSISSIPPI RIVER BRIDGE – TRUSS PIN AND

LINK REPLACEMENT

Introduction

The westbound US 84 Mississippi River Bridge

is a 5 span cantilever truss with a 7 span

approach that carries US 84 over the Mississippi

River between Natchez, Mississippi and Vidalia,

Louisiana (Figure 1). The Westbound Bridge

was designed by HNTB in 1939 and opened to

traffic in September 1940. The bridge was the

third Mississippi River Bridge built south of

Memphis, Tennessee and the first highway only

Mississippi River Bridge south of St. Louis,

Missouri. The bridge has one suspended span

located between Piers 1 and 2 and eyebar links

on Spans 2, 3, and 4. The Louisiana approaches,

Spans 6 through 11, are plate girders. A twin

structure located downstream was completed in

July 1988 and the older bridge was restriped to

two westbound lanes.

Figure 1 – Bridge Location Map

The westbound bridge has a 24’-0” clear

roadway width, 2’-0¼” wide steel curb and rail,

and a 7¼” thick deck (Figure 3). The deck is

supported by crossbeams which are supported

on 3 stringers. The stringers are framed into the

floor beams that are spaced at either 39’-10½“or

43’-9”. Eyebar links (2’-0” x 10” x 7’-6½” long)

are located at truss Joints U19, U29, U49, and

U69. The westbound bridge was originally

designed for a H15 vehicle (truck or lane).

A general elevation view is shown in Figure 2

and a section view is shown in Figure 3.

Figure 2 –General Elevation View

Figure 3 – Section View

As a border bridge between Louisiana and

Mississippi, the bridge is maintained by

Mississippi Department of Transportation

(MDOT) but cost equally shared between

MDOT and Louisiana Department of

Transportation and Development (DOTD).

Pin and Link Details

Unlike other cantilever truss bridges in which

eyebars supporting a suspended span are full

length truss member, the pins and links on the

US 84 Mississippi River Bridge are confined to

the upper joint. The suspended span and quasi-

Natchez, MS

New Orleans, LA

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suspended span loads pass through the

suspended span or quasi-suspended span upper

gusset and into the 7’-6½” long links via 101/16”

diameter lower pins (Figure 4). The load is then

transferred from the link into the cantilever span

upper gusset via 101/16” upper pins. The pins and

link also function as expansion joints for the

bridge and were designed for up to 9” of

movement between the two gussets.

Figure 4 – Upper Joint

Pin Movement

U29 Pin Movement

In 1995, MDOT observed the tie rod that holds

and restraints the pin from lateral movement was

fractured and cover plates missing on the lower

pin at U29 downstream truss. The weld that

prevents the lower pin from rotating about the

gusset was broken and the pin had rotated 2¾”

from its installed position. The lower pin was

also flush with the inside face exterior gusset on

one side and extended 1” on the outside face

exterior gusset (Figure 5 & 6).

MDOT contracted with HNTB which advised

the outside ½” gusset supported the hanger and

if the pin continued to move past the outside

gusset, there would be an adverse effect to the

factor of safety for the bridge, potentially

resulting in closure of the bridge.

Figure 5 – U29 Lower Pin Downstream Truss

Inside Face

Figure 6 – U29 Lower Pin Downstream Truss

Outside Face

Pin Rehabilitation

In 1996, MDOT awarded a contract to

temporarily remove the load off the pin and link

via temporary restraints and push the lower pin

back into place. Beneath one lane of traffic,

HNTB proposed a vertical jacking assembly that

would bypass the load on the link and pins via 4

post-tensioning bars (Figure 7).

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Figure 7 – Vertical Jacking Assembly

Prior to installing the vertical jacking assembly,

the contractor attempted to reset the pin without

removing the vertical load. The first attempt the

contractor applied 675 kips of horizontal load in

which his post-tensioning system failed resulting

in post-tensioning bars passing traffic and into

the river. The second attempt (Figure 8) the

contractor redesigned the horizontal jacks and

increased the horizontal load to 884 kips. The

third attempt the contractor redesigned the

horizontal jacks and increased the load to 1,325

kips, at which point no movement in the lower

pin was recorded.

Figure 8 – Horizontal Jacks

The fourth attempt, the contractor redesigned the

temporary restraints to include a vertical jacking

assembly in which he applied 800 kips of

vertical load to remove the theoretical dead load

in the pin and link and then applied 727 kips of

horizontal load to the pin, at which point no

movement in the lower pin was recorded.

Ultimately, MDOT and HNTB agreed any

additional attempts would be futile, potentially

cause damage to the bridge, and agreed to

regularly monitor the pin for additional

movement.

2010 In-Depth Inspection

In 2010 HNTB was contracted by MDOT to

complete an in-depth inspection of the

westbound US 84 Mississippi River Bridge in

which non-destructive testing was completed on

8 of the 16 pins. The non-destructive testing

revealed inter-component acoustic coupling

(ICAC) between the lower pin at U29

downstream truss and the link which indicated

the pin may have fused with the link. ICAC

typically occurs when the ultrasonic wave from

an ultrasonic examination is reflected from the

transverse surface of an adjacent component,

typically under high local bearing stress. The

lower pin was still flush with the exterior gusset

but an oblong hole in the gussets with a 3/8” gap

between the bottom of the lower pin and the

gusset was observed. The oblong hole was

consistent with the assumption the lower pin was

rotating about the gusset vs. the pin. It was the

intention of the original designer that the pin

would not rotate about the gusset and the

bearing stress on the gusset from the pin be 0.56

Fy. (AASHTO allows 0.4 Fy for pins subject to

rotation and 0.8 Fy for pins not subject to

rotation)

The in-depth inspection also revealed the lower

pin tie rod on U49 upstream truss had fractured,

the cover plates missing, the pin was flush with

the exterior gusset, and there was roughly ½”

gap between the bottom of the lower pin (Figure

9).

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Figure 9 – U49 Lower Pin

Pin and Link Replacement

Investigation

After the 2010 in-depth inspection, MDOT

contracted with HNTB to investigate and make

recommendations to address the lower pins at

U29 downstream truss and U49 upstream truss.

HNTB investigated four options:

1. Restrain and monitor

2. Rest pins

3. Replace lower pin

4. Replace lower and upper pins and link

Option 1 – Restrain and Monitor – This option is

similar to the “no build” option in an

environmental assessment and entails

reinstalling the cover plates on the lower pins

and continuing to monitor.

Option 2 – Reset Pins – This option would entail

reusing the concept from 1996 and attempting to

reset the lower pins

Option 3 – Replace Lower Pins – This option

would entail using the vertical jacking assembly

similar to figure 7, however, using destructive

measures to remove the lower pin, boring a new

hole in the gusset and link, and installing a new

lower pin.

Option 4 – Replace Lower and Upper Pins and

Link – This option would entail installing

temporary restraints so that the upper and lower

pins and link could be removed and replaced.

Risk Matrix

In order to review all four options, HNTB

prepared a risk matrix for all four options listing

the pros and cons to each option for MDOT and

DOTD to complete. The risk matrix listed risk

and probability/likelihood on a scale of 1 to 5 for

the different options and components within

each option.

Option 1 – Although MDOT had been

monitoring U29 for over 15 years, this option

represented the highest risk with moderate

probability. With the bridge at its design life of

75 years, the pins could have shifted for several

reasons such as wear or pier movement.

Unfortunately, there was minimal information to

support or dismiss theories. Ultimately, if the

pin moved further within a 12 month period or

became locked, there would be little to no

warning signs outside of complete collapse. The

probability was identified as moderate due to

fact the lower pin at U29 downstream truss had

not moved in 15 years, however, the pin at U49

upstream truss had. MDOT and DOTD decided

this option was not preferred.

Option 2 – If successful, this option would

represent the lowest risk; however, it was

assigned a low probability of being successful.

Based on the experience in 1996, the contractor

was unsuccessful at resetting the lower pin at

U29 and the non-destructive testing noted

acoustic coupling between the lower pin and link

which indicated potential fusing. If fused, the

pin was not designed to rotate about the gusset

which can be observed by the oblong hole in the

gusset from the lower pin wear. The other lesson

learned from the 1996 attempt was the fact the

pin must be rotated prior to pushing back. It is

anticipated the pin has grooves, and similar to a

key in a lock, unless the pin is rotated while

being pushed, any attempts would be futile.

MDOT and DOTD decided this option was not

preferred.

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Option 3 - HNTB completed a comprehensive

investigation of option 3 but the risk of

damaging or finding damage on the existing link

proved too high. Although the probability of

damage on the existing link was low, the links

are unable to be tested and are at their design

life. Visual inspections have been limited due to

special constraints and key sections would not

be visible until the lower pin was removed.

Contingency plans were contemplated in the

event the links needed to be replaced; however,

MDOT and DOTD decided this option was not

preferred.

Option 4 – MDOT and DOTD unanimously

agreed replacing the upper and lower pins and

the link at U29 downstream truss and U49

upstream truss was the preferred option. This

option had the highest probability of being

successful with risk that could be mitigated

through the design of HNTB’s temporary

restraints.

Pin and Link Replacement

In order to remove the pins and link, a

temporary bypass that locks the joint from

moving in all directions was developed. It was

important the temporary bypass had internal

redundancy plus alternate load paths to mitigate

the risk of any one component compromising the

bridge when the pins and link were removed. A

series of bypasses were used to lock the joint

and the Pier was expected to flex under thermal

loads.

The temporary restraints were comprised of four

main components; Upper Longitudinal Restraint,

Diagonal Bypass, Lower Longitudinal Restraint,

and Splice Plate (Figure 10).

Figure 10 – Temporary Restraints

Upper Longitudinal Restraint (Figure 11) – Due

to the fact U29 and U49 are expansion joints, the

pins and link are free to rotate. When removing

the link, the two gussets must be locked

together. The upper longitudinal restraints use

post-tensioning bars plus shim blocks to

compress the two gussets together until the

splice plate is installed. The upper longitudinal

restraints are applied to both upstream and

downstream trusses when removing the pins and

links.

Figure 11 – Upper Longitudinal Restraints

Diagonal Bypass (Figure 12 & 13) –The

majority of the load in the link is from the

diagonal truss member on the suspended span

(lower pin side of the gusset). The diagonal

bypass was designed to unload the suspended

span diagonal truss member and link. Once

installed, the suspended span would bypass the

lower pin and be transferred into the cantilever

span gusset from above.

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Figure 12 – Diagonal Bypass

Figure 13 – Diagonal Bypass

Lower Longitudinal Restraint (Figure 14) – L29

and L49 bottom chord truss members currently

are false chord members; however, they have

similar properties to the other truss members.

The Lower Longitudinal Restraint connects the

two bottom chord members allowing them to act

as an alternate load path. Shims were installed

between the two members and post-tensioned to

ensure they remained in compression.

Figure 14 – Lower Longitudinal Restraints

Temporary Splice Plate (Figure 15) – As a

means to control displacement and provide an

alternate load path, a splice plate was designed

to connect the suspended span gusset to the

cantilever span gusset. The temporary splice

plate required over 350 A490 bolts per truss

which entailed removing existing rivets in the

gusset or field drilling new holes. Because only

one rivet could be removed at a time, each bolt

was installed with a custom nut between the

gusset and splice plate to ensure the splice plate

bore uniformly on the middle nuts and middle

nuts on the gussets. The force in the each bolt

was calculated and bending in the bolt checked.

Cheek or shim plates were installed between the

gussets and splice plate to provide additional

friction force, however, the friction force was

not included in the design of the splice plate or

bolts.

Figure 15 – Temporary Splice Plate

Redundant Load Path

In order to mitigate risk while the pin and link

are removed, the temporary restraints were

designed so that they had internal redundancy as

well as additional load paths in the event one

system is lost. Figure 16 and 17 & 18 illustrate

the three load paths.

Figure 16 – Temporary Load Path A – Diagonal

Bypass

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Figure 17 – Temporary Load Path B – Splice

Plate and Lower Longitudinal Restraint

Figure 18 – Temporary Load Path C – Splice

Plate

Construction

Overview

Typically MDOT projects are either design-bid-

build or design-build. In the design-bid-build

option, MDOT or its consultant prepares a set of

plans and the project is advertised and any

contractor may place a bid with the low bid

winning. MDOT and FHWA agreed that due to

unique nature of the work, plus the risk of a

mistake or carelessness could result in collapse

or severe damage to the bridge, MDOT decided

to advertise a design-bid-build with a two-step

process. Step one consisted of a request for

qualification from contractors with a short list of

qualified contractors. Two qualified contractors

were shortlisted with both submitting bids based

on plans developed by HNTB. C.E.C. out of

Lafayette, Louisiana was the low bidder at $3.8

million and awarded the project.

Traffic Control

It was decided that because of the risk associated

with removing the pin and link, the traveling

public should not be on the bridge during the

replacement. MDOT wisely decided to install

crossovers and put traffic head to head on the

eastbound bridge as well as re-synchronize

traffic lights. Minimal to no queue was

observed throughout the duration of the project.

U29 Misalignment

During installation of temporary restraints, it

was observed that U29 upper pin gusset

(cantilever span gusset) and lower pin gusset

(suspended span gusset) near the lower pin were

shifted inboard by 1.875” where they should

have been centered with each other (Figure 19).

The cantilever span gusset near the upper pin

was shifted by ¾” and the cantilever span gusset

appeared to have a slight rotation. Truss

member U29-L30 also was kinked near the

connection point at L30 (Figure 20). Based on

review of the 1940 construction and erection

records of the bridge, the suspended span from

L22 was cantilevered out with the final tie-in

between the suspended and the cantilever span at

U29 (Figure 21). The misalignment was

indicative of a geometric misalignment during

erection of the bridge and the two spans were

pulled together laterally in order to install the

pins and link. It was speculated that U29 had

locked-up lateral erection force which was being

restrained by the existing pins and link.

In order to mitigate the locked-up erection

forces, the anticipated load was calculated based

on the observed deflection and checked against

the top strut lateral restraints and found to be

satisfactory. 200 plus additional A490 bolts

were required at U29 splice plate to mitigate the

additional bending plus an interior plate that

engaged the entire bolt group installed. (Figure

22)

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Figure 19 – U29 Offset

Figure 20 – U29-L30 Kink

Figure 21 – 1940 Construction

Figure 22 – U29 Middle Plate

Instrumentation

In order to ensure the temporary restraints were

properly transferring the load off the pin and

link as well as to evaluate any unforeseen losses

in the restraints, strain gauges were installed on

multiple truss members and the post-tensioning

bars. The splice plates were also instrumented

to evaluate stresses once the pins were removed.

Because the temporary restraints would change

the boundary conditions of the bridge to fixed-

fixed, adding additional load in the truss and

forcing the Piers to flex, the Piers were inspected

prior to and after locking each joint.

The initial inspection of the Piers revealed

numerous cracks, as is expected for a mildly

reinforced Pier at 75 years of age. No crack

growth was observed in the post-inspection.

Based on the instrumentation output from the 8

links over a two week period it was obvious the

bridge was behaving in a fixed-fixed condition.

The existing link and pins would build up as

much as 3.0 ksi of stress before breaking free

and equalizing back to zero.

Sequence of Construction

As part of the contract plans, the contractor was

required to submit a detailed sequence of

construction demonstrating means and methods

for removing the pin and link. The contract

plans provided a suggested sequence of

construction in which the contractor adopted

with minor modifications. The contractor chose

to remove the pins and link at U49 first, and

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based on lessons learned, some adjustments

were made at U29. The following are the key

steps to the sequence of construction with

lessons learned from construction:

Step 1 – Tension Diagonal Bypass (Figure 23

and 24). L48 – U49 diagonal bypass was

tensioned to remove the load in the existing truss

diagonal member, link and pin. Stressing

operation were conducted in increments and

member stresses observed to ensure the bypass

was functioning as anticipated. U49 existing

link change in force was monitored and U49

diagonal bypass was tensioned to 10% over the

anticipated force, resulting in the change in force

in the link to be within 1% of the anticipated

load (Table 1). Although the entire load would

not be released until the pins were removed, it

was preferred to minimize the load in the

existing link to avoid the pins from binding and

prevent sudden movement resulting from pin

removal.

Figure 23 – Tensioning of Diagonal Bypass

Figure 24 – Tensioning of diagonal bypass

Table 1 – Diagonal Bypass

Member Load Anticipated

Load

U49 Diagonal

Bypass

836 kips 760 kips

L48-U49 580 kips 761 kips

U49 Link 660 kips 655 kips

U29 Diagonal

Bypass

782 kips 740 kips

L28-U29 699 kips 740 kips

U29 Link 634 kips 640 kips

Prior to tensioning U49 diagonal restraint, upper

and lower shims were installed. The upper

shims would transfer any horizontal force in the

diagonal restraint. The lower shims would not

be required until Step 2 but were chosen to be

installed during Step 1 (Figure 25 and 26).

During the stressing of the diagonal bypass, the

lower shim shifted transversely by ¼”. As the

diagonal restraints became fully engaged, it

became clear that one of the post-tensioning bars

was conflicting with the truss lateral bracing,

potentially causing the shift at the lower shims.

The portion of truss lateral bracing that was in

conflict was cut further allowing the diagonal

post-tensioning bars to adjust; however, because

the upper bypass was engaged, the horizontal

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load from the upper bypass did not allow the

shims between the two gussets to shift back

horizontally.

Once the existing pin was removed and the new

one installed, it was observed that the inboard

and outboard lower gusset had walked out by

½”. Fortunately the new lower pin was made

longer and the pin bore directly on the gussets;

however, the pin extension beyond the gusset

was minimal. Although the gussets did not

appear to walk when engaging the diagonal

restraints, the existing pins may have been

restraining them, and once removed, the gussets

were free to walk.

For U29, the lower shims were not installed until

after the diagonal bypass was engaged and prior

to tensioning the upper longitudinal restraint.

An additional stiffener plate was also installed

and as a result, U29 outboard and inboard gusset

did not walk.

Figure 25 – Upper Shims

Figure 26 – Lower Shims

Step 2 – Tension Upper Longitudinal Restraint

(Figure 27 and 28). Both upstream truss and

downstream truss upper longitudinal restraints

were tensioned to prevent the joint from moving

longitudinally. The upper longitudinal restraints

were designed for a 60 degree temperature drop

but stressed to accommodate a 40 degree

temperature drop based on the 10 day weather

forecast.

Figure 27- Upper Longitudinal Restraint

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Figure 28- Upper Longitudinal Restraint

Step 3 – Weld Templates and Field Drill Splice

Plate (Figure 29 and 30). Once the bridge was

locked from moving, the splice plate templates

were welded together and used to field drill the

splice plates. Field drilling and installing the

splice plates was challenging due to the 100 plus

A490 bolts per face of gusset, but was

completed with minimal to no incident.

Figure 29 – Splice Plate Template

Figure 30 – Field Drill Splice Plate

Step 4 – Install Top Strut Plates (Figure 31).

Although there was a wind shear device at U29

and U49, the two top strut were connected

together to provide lateral additional rigidity in

the event there were any unexpected lateral

forces when the link was removed.

Figure 31 – Top Strut Plate

Step 5 – Install Lower Longitudinal Restraints

(Figure 32 and 33). Shims were installed

between the two false chord members and post-

tensioned together to ensure continuous bearing

between members.

Figure 32 – Lower Longitudinal Restraint Shims

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Figure 33 – Lower Longitudinal Restraints

Step 6 – Remove Pins (Figure 34 and 35).

Because of the difficulty the previous contractor

had with trying to reset the pins in 1997, the

contractor elected to cut the pin with a diamond

tipped wire saw. After cutting U49 upper pin,

minimal change in force was observed in the

link, and U49 and U29 splice plates saw about 1

ksi and 7.5 ksi of stress respectively (Table 2).

It was speculated the higher stress in U29 was

attributed to the misalignment of the truss. No

movement was observed in either joint during

removal of the pins.

The contractor attempted to push out U49 lower

pin with hydraulics jacks after the upper pin was

cut, but after applying 1,000 kips, minimal to no

movement was observed. Ultimately, both faces

of all pins required cutting and after the lower

pins were removed, it became clear from the

observed amount of grooving, the pins would

not been able to be pushed out (Figure 36).

Figure 34 – Wire Saw

Figure 35 – Wire Saw Inside Truss

Figure 36 – U49 Lower Pin Drop Cut

Table 2 - U29 Splice Plate Maximum Stress

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Step 7 – Line Bore (Figure 37). The contractor

line bored a 10½” to 10¾” hole through the

existing gusset and new eyebars to ensure the

new pins would bear properly and fit. With the

existing link and pins removed, it was found that

the existing upper pins at U29 and U49 were not

plumb whereas the lower pins were. It is

speculated this may have been part of the cause

of the walking observed at the existing lower

pins. It was also noted that U29 inboard and

outboard gussets were not plumb and this was

attributed to the locked-up erection forces in the

gusset. New pins were bored plumb and in line

with each other.

Figure 37 – Line Bore of Upper Pin

Step 8 – Install New Pins (Figure 38). Once the

line bore was complete, the new pins were able

to be installed with little difficulty. For the first

location, U49, all four pins were machined and

on site. However with the existing upper pin

hole not being plumb and new hole required to

be plumb, the existing upper original diameter of

10¼” was inadequate. The contractor sent U29

lower pin, which was 10¾” diameter, back to the

machine shop to have it turned down to the

needed diameter of 10½”.

New pins were ordered for U29, however, they

were not turned down until after the line boring

was 50% complete.

Figure 38 – Installation of New Pins

Conclusion

After the new pins were installed, the temporary

restraints were disengaged and load transferred

to the new pins and eyebars. Table 3 includes

the results from U29 and U49 eyebars. U49

eyebar loads appeared to be symmetric and

behaved as a deep beam governed by Euler-

Bernoulli. The inside eybar was 15% greater

than the outside eyebar and it is thought that the

inside gusset may carry more load due to the fact

that the weight of roadway is transferred through

the floor system which favors the inside gusset.

U29 loads were not as symmetric as U49 and

heavily favor the outside gusset. The outboard

eyebar was 44% greater than the inboard eyebar

and it is thought that the misalignment in U29

was the primary culprit for the imbalance.

When comparing the total loads in Table 3, both

U29 and U49 were within 3% of the dead loads

shown on the 1940 contract plans.

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Table 3 – U29 and U49 Eyebar loads

Location U29 U49

Bar 1 (inboard) 70.9 kips 135.3 kips

Bar 2 105.4 kips 115.9 kips



Bar5 125.6 kips 104.1 kips

Bar 6 (outboard) 127.0 kips 115.7 kips

Total 656.5 kips 673 kips

DL in 1940 Plans 638 kips 655 kips

MDOT, HNTB and CEC all felt the project was

a success and MDOT is anticipating replacing

the remaining 12 pins and 6 links on a future

project. The primary key to success was the

partnership and determination between all the

parties involved to make the project successful.

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